Tomography is a method of imaging in which slice images of a three-dimensional recording area are reconstructed. A tomography device has a recording unit with a central system axis. The recording unit can be embodied in a ring shape or in a tunnel shape. The recording unit also has an isocenter, in which the conditions for a tomographic recording are especially advantageous. Typically system axis and isocenter are made to coincide at least partly. The recording area can be moved during the tomographic recording along the system axis and thus through the isocenter. At the end of the tomographic recording the projections are processed in such a way that a tomographic image is produced.
In the case of x-ray tomography x-ray projections are recorded from different projection angles. During this process the recording unit, with a radiation source and also a radiation detector, rotates around the system axis and also around the recording area. The intersection point between the rays emitted by the radiation source and the system axis forms the isocenter of the recording unit. In magnetic resonance tomography the radiation detector can especially be disposed in the form of local coils outside the recording unit. Furthermore in magnetic resonance tomography the system axis is disposed in parallel to a main magnetic field, wherein the isocenter is characterized by an especially homogeneous main magnetic field.
How the recording area is positioned is decisive for the quality of a tomographic image. For example it is regularly desirable to position the radiological focus of the recording area or of an examination area lying in the recording area in the isocenter of the recording unit of a tomography device. The result of this is that the radiation is attenuated as evenly as possible. An exact positioning is especially important in the clinical environment when the recording area involves a region of a patient's body. This is because repeating a tomographic recording as a result of mispositioning involves an additional radiation load and a significant delay in day-to-day clinical procedures. Furthermore a tomographic image of the highest possible quality is indispensible in clinical diagnostics.
Conventionally the patient is positioned by a user moving the patient couch manually. An optical marking, which is projected onto the patient, typically in the form of a laser line, is also used for positioning. A particular problem in such cases is the positioning of the patient perpendicular to the system axis, especially the positioning in the vertical direction. Because of the great time pressure in everyday clinical procedures, the vertical position of the patient in particular is often set only insufficiently accurately.